Synchronously-switched multi-input demodulating comparator

ABSTRACT

Methods and systems are described for obtaining a set of carrier-modulated symbols of a carrier-modulated codeword, each carrier-modulated symbol received via a respective wire of a plurality of wires of a multi-wire bus, applying each carrier-modulated symbol of the set of carrier-modulated symbols to a corresponding transistor of a set of transistors, the set of transistors further connected to a pair of output nodes according to a sub-channel vector of a plurality of mutually orthogonal sub-channel vectors, recovering a demodulation signal from the carrier-modulated symbols, and generating a demodulated sub-channel data output as a differential voltage on the pair of output nodes based on a linear combination of the set of carrier-modulated symbols by controlling conductivity of the set of transistors according to the demodulation signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/909,529, filed Jun. 23, 2020, naming Armin Tajalli, entitled“Synchronously-Switched Multi-Input Demodulating Comparator”, which is acontinuation of U.S. application Ser. No. 16/236,012, filed Dec. 28,2018, now U.S. Pat. No. 10,693,688, naming Armin Taj alli, entitled“Synchronously-Switched Multi-Input Demodulating Comparator”, whichclaims the benefit of U.S. Provisional Application No. 62/611,523, filedDec. 28, 2017, naming Armin Tajalli, entitled “Combined Multi-InputComparator and Demodulator”, all of which are hereby incorporated byreference in their entirety for all purposes.

REFERENCES

The following references are herein incorporated by reference in theirentirety for all purposes:

U.S. Patent Publication No. 2011/0268225 of U.S. patent application Ser.No. 12/784,414, filed May 20, 2010, now U.S. Pat. No. 9,288,089, grantedMar. 15, 2016, naming Harm Cronie and Amin Shokrollahi, entitled“Orthogonal Differential Vector Signaling”, hereinafter identified as[Cronie I];

U.S. patent application Ser. No. 13/030,027, filed Feb. 17, 2011, nowU.S. Pat. No. 8,649,445, granted Feb. 11, 2014, naming Harm Cronie, AminShokrollahi and Armin Tajalli, entitled “Methods and Systems for NoiseResilient, Pin-Efficient and Low Power Communications with SparseSignaling Codes”, hereinafter identified as [Cronie II];

U.S. patent application Ser. No. 14/158,452, filed Jan. 17, 2014, nowU.S. Pat. No. 9,124,557, granted Sep. 1, 2015, naming John Fox, BrianHolden, Peter Hunt, John D Keay, Amin Shokrollahi, Richard Simpson,Anant Singh, Andrew Kevin John Stewart, and Giuseppe Surace, entitled“Chip-to-Chip Communication with Reduced SSO Noise”, hereinafteridentified as [Fox I];

U.S. patent application Ser. No. 13/842,740, filed Mar. 15, 2013, nowU.S. Pat. No. 9,300,503, granted Mar. 29, 2016, naming Brian Holden,Amin Shokrollahi and Anant Singh, entitled “Methods and Systems for SkewTolerance in and Advanced Detectors for Vector Signaling Codes forChip-to-Chip Communication”, hereinafter identified as [Holden I];

U.S. Provisional Patent Application No. 61/934,804, filed Feb. 2, 2014,now U.S. patent application Ser. No. 14/612,241, filed Feb. 2, 2015,U.S. Pat. No. 9,100,232, granted Aug. 6, 2015, naming Ali Hormati andAmin Shokrollahi, entitled “Methods for Code Evaluation Using ISIRatio”, hereinafter identified as [Hormati I];

U.S. Provisional Patent Application No. 62/026,860, filed Jul. 21, 2014,now U.S. patent application Ser. No. 14/803,996, filed Jul. 20, 2015,U.S. Pat. No. 9,444,654, granted Sep. 13, 2016, naming Ali Hormati andAmin Shokrollahi, entitled “Multidrop Data Transfer”, hereinafteridentified as [Hormati II];

U.S. patent application Ser. No. 15/194,497, filed Jun. 27, 2016, nowU.S. Pat. No. 9,832,046, granted Nov. 28, 2017, naming Ali Hormati,Armin Tajalli, and Amin Shokrollahi, entitled “Method and Apparatus forHigh-Speed Chip-to-Chip Communications”, hereinafter identified as[Hormati III];

U.S. patent application Ser. No. 15/802,365, filed Nov. 2, 2017, nowU.S. Pat. No. 10,347,283, granted Jul. 9, 2019, naming Ali Hormati andArmin Tajalli, entitled “Clock Data Recovery in Multilane DataReceiver”, hereinafter identified as [Hormati IV].

U.S. Provisional Patent Application No. 61/934,807, filed Feb. 2, 2014,now U.S. patent application Ser. No. 14/612,252, filed Feb. 2, 2015,U.S. Pat. No. 9,479,369, granted Oct. 25, 2016, naming Amin Shokrollahi,entitled “Vector Signaling Codes with High pin-efficiency and theirApplication to Chip-to-Chip Communications and Storage”, hereinafteridentified as [Shokrollahi I];

U.S. Provisional Patent Application No. 61/839,360, filed Jun. 23, 2013,now U.S. patent application Ser. No. 14/313,966, filed Jun. 24, 2014,U.S. Pat. No. 9,246,713, granted Jan. 26, 2016, naming Amin Shokrollahi,entitled “Vector Signaling Codes with Reduced Receiver Complexity”,hereinafter identified as [Shokrollahi II].

U.S. Provisional Patent Application No. 61/946,574, filed Feb. 28, 2014,now U.S. patent application Ser. No. 14/636,098, filed Mar. 2, 2015,U.S. Pat. No. 9,363,114, granted Jun. 7, 2016, naming Amin Shokrollahi,Brian Holden, and Richard Simpson, entitled “Clock Embedded VectorSignaling Codes”, hereinafter identified as [Shokrollahi III].

U.S. Provisional Patent Application No. 62/015,172, filed Jul. 10, 2014,now U.S. patent application Ser. No. 14/746,477, filed Jun. 22, 2015,U.S. Pat. No. 9,852,806, granted Dec. 26, 2017, naming Amin Shokrollahiand Roger Ulrich, entitled “Vector Signaling Codes with Increased Signalto Noise Characteristics”, hereinafter identified as [Shokrollahi IV].

U.S. patent application Ser. No. 13/895,206, filed May 15, 2013, nowU.S. Pat. No. 9,288,082, granted Mar. 15, 2016, naming Roger Ulrich andPeter Hunt, entitled “Circuits for Efficient Detection of VectorSignaling Codes for Chip-to-Chip Communications using Sums ofDifferences”, hereinafter identified as [Ulrich I].

“Controlled Intersymbol Interference Design Techniques of ConventionalInterconnection Systems for Data Rates beyond 20 Gbps”, WendemagegnehuT. Beyene and Amir Amirkhany, IEEE Transactions on Advanced Packaging,Vol. 31 No. 4, pg. 731-740, November 2008, hereinafter identified as[Beyene].

TECHNICAL FIELD

The present invention relates to communications in general and inparticular to the transmission of signals capable of conveyinginformation and detection of those signals in wired communication.

BACKGROUND

In communication systems, a goal is to transport information from onephysical location to another. It is typically desirable that thetransport of this information is reliable, is fast and consumes aminimal amount of resources. Methods of information transport arebroadly categorized into “baseband” methods that dedicate use of thephysical communications channel to one transport method, and “broadband”methods that partition the physical communications channel in thefrequency domain, creating two or more independent frequency channelsupon which a transport method may be applied.

Baseband methods may be further categorized by physical medium. Onecommon information transfer medium is the serial communications link,which may be based on a single wire circuit relative to ground or othercommon reference, multiple such circuits relative to ground or othercommon reference, or multiple such circuits used in relation to eachother. A common example of the latter uses differential signaling(“DS”). Differential signaling operates by sending a signal on one wireand the opposite of that signal on a matching wire. The signalinformation is represented by the difference between the wires, ratherthan their absolute values relative to ground or other fixed reference.

Parallel data transfer is also commonly used to provide increasedinterconnection bandwidth, with busses growing from 16 or fewer wires,to 32, 64, and more. As crosstalk and noise induced on the parallelsignal lines can produce receive errors, parity was added to improveerror detection, and signal anomalies were addressed through active bustermination methods. However, these wide data transfer widths inevitablyresulted in data skew, which became the limiting factor in increased busdata transfer throughput. Alternative approaches were developedutilizing narrower bus widths operating at much higher clock speeds,with significant effort placed on optimizing the transmission linecharacteristics of the interconnection medium, including use ofimpedance-controlled connectors and micro stripline wiring. Even so, theinevitable path imperfections required use of active equalization andinter-symbol interference (ISI) elimination techniques, including activepre-emphasis compensation for transmitters and Continuous Time LinearEqualization (CTLE) and Decision Feedback Equalization (DFE) forreceivers, all of which increased the complexity and power consumptionof the communications interface.

A number of signaling methods are known that maintain the desirableproperties of DS, while increasing pin efficiency over DS. One suchmethod is Vector signaling. With vector signaling, a plurality ofsignals on a plurality of wires is considered collectively although eachof the plurality of signals might be independent. Thus, vector signalingcodes can combine the robustness of single circuit DS and the high wirecount data transfer throughput of parallel data transfer. Each of thecollective signals in the transport medium carrying a vector signalingcodeword is referred to as a component, and the number of plurality ofwires is referred to as the “dimension” of the codeword (sometimes alsocalled a “vector”). With binary vector signaling, each component or“symbol” of the vector takes on one of two possible values. Withnon-binary vector signaling, each symbol has a value that is a selectionfrom a set of more than two possible values. The set of values that asymbol of the vector may take on is called the “alphabet” of the vectorsignaling code. A vector signaling code, as described herein, is acollection C of vectors of the same length N, called codewords. Anysuitable subset of a vector signaling code denotes a “subcode” of thatcode. Such a subcode may itself be a vector signaling code. Inoperation, the coordinates of the codewords are bounded, and we chooseto represent them by real numbers between −1 and 1. The ratio betweenthe binary logarithm of the size of C and the length N is called thepin-efficiency of the vector signaling code. A vector signaling code iscalled “balanced” if for all its codewords the sum of the coordinates isalways zero. Additional examples of vector signaling methods aredescribed in Cronie I, Cronie II, Cronie III, Cronie IV, Fox I, Fox II,Fox III, Holden I, Shokrollahi I, Shokrollahi II, and Hormati I.

As previously described, broadband signaling methods partition theavailable information transfer medium in the frequency domain, creatingtwo or more frequency-domain “channels” which may then may transportinformation in a comparable manner to baseband circuits, using knownmethods of carrier modulation to convert the baseband information into afrequency-domain channel signal. As each such channel can beindependently controlled as to amplitude, modulation, and informationencoding, it is possible to adapt the collection of channels to widelyvarying information transfer medium characteristics, includingvariations in signal loss, distortion, and noise over time andfrequency.

Asymmetric Digital Subscriber Line or ADSL is one widely deployedbroadband signaling method used to transport digital data over legacycopper telephony circuits. In ADSL, each of potentially several hundredfrequency-domain channels is independently configured for amplitude,modulation method, and digital carrying capacity, based on theparticular noise and loss characteristics of the copper circuit beingused for transport.

BRIEF DESCRIPTION

Methods and systems are described for obtaining a set ofcarrier-modulated symbols of a carrier-modulated codeword, eachcarrier-modulated symbol received via a respective wire of a pluralityof wires of a multi-wire bus, applying each carrier-modulated symbol ofthe set of carrier-modulated symbols to a corresponding transistor of aset of transistors, each transistor of the set of transistors connectedto a respective output node of a pair of output nodes according toelements of a sub-channel vector, and controlling conductivity of theset of transistors according to a demodulation signal operating at afrequency recovered from the carrier-modulated symbols to responsivelygenerate a demodulated sub-channel data output as a linear combinationof the set of carrier-modulated symbols forming a differential voltageon the pair of output nodes.

Communication of digital information using a combination of baseband andbroadband techniques over multiple wires is described. A four wirecommunications channel having 35 dB of attenuation at 37.5 GHz is usedin provided examples as a typical transport medium for use with thesystems and methods described herein. One embodiment creates twofrequency-based channels over the transport medium, with each channelusing a combination of a vector signaling code and duobinary encoding totransport sets of three data bits over four wires at an effective rateof 56 Gigabits per second per wire.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a block diagram of a receiver embodiment.

FIG. 2 is a schematic of one embodiment of a circuit detecting one ENRZsubchannel.

FIG. 3 is a schematic of another embodiment of a circuit detecting oneENRZ subchannel.

FIG. 4 is a schematic of one embodiment of a circuit detecting onecarrier-modulated ENRZ subchannel.

FIG. 5 is a schematic of another embodiment of a circuit detecting onecarrier-modulated ENRZ subchannel.

FIG. 6 is a schematic of a further embodiment of a circuit detecting onecarrier-modulated ENRZ subchannel.

FIG. 7 illustrates waveforms for the carrier-modulated data encoding anddecoding operations.

FIGS. 8A and 8B illustrate alternative embodiments for receiver clockgeneration.

FIG. 9 is a flowchart of a method, in accordance with some embodiments.

DETAILED DESCRIPTION

Interconnection has long been a limiting factor in the design of largedigital systems. Whether at the level of modules interconnected by abackplane, or of functional subsystems interconnected within a largeprinted circuit board, the need for reliable, error free, high-speeddigital interconnection has constantly pushed the limits of availabletechnology to its limits.

The systems and methods described herein provide robust, reliabletransfer of data between at least one transmitting device and at leastone receiving device, at data rates of at least 50 Gigabits per secondper interconnection wire. An example channel model having the frequency-and time-domain characteristics illustrated in FIG. 1 will be used. Itwill be obvious to one familiar with the art that such a transportchannel is incompatible with conventional communication signalingmethods; for example, straightforward NRZ signaling at an example 112Gibabits/second has a Nyquist frequency of 56 GHz, corresponding to anintractable 46 dB attenuation over the proposed physical transportchannel.

This proposed data rate also strains integrated circuit data processingcapabilities within the attached transmitting and receiving devices. Itis therefore presumed that high-speed data handling in these deviceswill be distributed across multiple parallel processing “phases”. As oneexample, rather than a single data path handling data at 100 Gigabitsper second (i.e. with merely 10 picosecond between bits), the same datastream may be distributed across sixteen processing phases, each onethus having a more reasonable 160 picoseconds of processing time perbit. However, this added processing time comes at the cost ofsignificantly increased complexity from the additional processingelements. This distribution of processing also can lead to increasedlatency before a given digital bit result becomes available, limitingthe ability to utilize that result in predicting a subsequent bitresult, which is the basis of the Decision Feedback Equalization or DFEmethod.

The increasing data transfer rates also lead to physical issues as thewavelength of the propagating signals on the interconnection shrinks. Asone example, the propagating signal wavelength at 56 Gigahertz on aprinted circuit micro stripline is approximately 4 millimeters, thusperiodic anomalies with merely fractional wavelength dimensions (evenincluding the weave of the impregnated fabric comprising the circuitboard) may represent a significant disturbance to signal integrity,stressing available equalization and compensation methods.

Encoding Information Using Hadamard Transforms

As taught in [Cronie I], the Hadamard Transform, also known as theWalsh-Hadamard transform, is a square matrix of entries +1 and −1 soarranged that both all rows and all columns are mutually orthogonal.Hadamard matrices are known for all sizes 2N as well as for selectedother sizes. In particular, the description herein utilizes the 4×4Hadamard matrix as the example encoder.

The order 4 Hadamard matrix used in our examples is:

$\begin{matrix}{H_{4} = \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1}\end{bmatrix}} & ( {{Eqn}.\mspace{14mu} 1} )\end{matrix}$

and encoding of the three informational bits A, B, C may be obtained bymultiplying those informational bits times the rows 2, 3, and 4 of theHadamard matrix H4 to obtain four output values, subsequently called“symbol values”. By convention, the results are scaled by an appropriateconstant factor so as to bound the symbol values to the range +1 to −1.It may be noted that the first row of H₄ corresponds to common modesignaling, which is not used herein, with the next three vectors beingused to encode bits A, B, and C respectively into outputs W, X, Y, Z,these vectors also being called “modes” or “subchannels” of the Hadamardcode. As the encoded outputs simultaneously carry information derivedfrom the encoding of A, B, and C, the outputs will be a superposition orsummation of modes, i.e. a sum of the sub-channel code vectors of thevector signaling code.

One familiar with the art will note that all possible values of A, B, Cencoded in this manner result in mode summed values for W, X, Y, Z whichare balanced; that is, summing to the constant value zero. If the modesummed values for W, X, Y, Z are scaled such that their maximum absolutevalue is 1 (that is, the signals are in the range +1 to −1 forconvenience of description,) it will be noted that all achievable valuesare permutations of the vector (+1, −1/3, −1/3, −1/3) or of the vector(−1, 1/3, 1/3, 1/3). These are called the codewords of the vectorsignaling code H4. As used herein, this H4 code will subsequently becalled Ensemble NRZ code or ENRZ and will be used as a representativeexample of vector signaling code in subsequent examples, withoutimplying limitation.

ENRZ

[Hormati I] teaches that ENRZ has optimum Inter Symbol Interference(ISI) characteristics, and [Holden I] and [Ulrich I] teach it is capableof efficient detection. As previously described, ENRZ encodes threebinary data bits into a four-symbol codeword for transmission, as oneexample, over four wires of a transport medium. If ENRZ signaling isused over four wires of the proposed channel, the data transfer rate maybe achieved with merely a 75 Gigasymbol/second signaling rate,equivalent to 112 Gbps per wire pair, for the two pair transportchannel.

Duobinary Encoding

Duobinary encoding is a solution known in the art in which consecutivebits of a serially transmitted data stream are processed to shape andconstrain the resulting transmit data spectrum. It is well known thatInter-Symbol Interference (ISI) such as may be produced by transmissionmedium perturbations will result in the received amplitude of a signalin one unit interval to be perturbed by residual energy from previousunit intervals. As one example, inverted pulse reflections from aperturbation of the transmission medium will cause a received signal tobe reduced by the residual influence of previously transmitted signals.Thus, a transmitter informed of this effect might combine a presentlytransmitted signal value with that of a previous transmission, in anattempt to anticipate or pre-compensate for this inter-symbolinterference effect. Thus, use of partial response codes such asduobinary are often described as a particular form of pre-equalizationfiltering intended to produce constructive ISI, rather than as a literaldata encoding means.

As described in [Beyene], other partial-response codes are known to havecomparable ISI management capabilities. For reference purposes, thecharacteristic equations defining these encodings or filterings arelisted in Table I.

Table I Partial Response System Characteristic Equation Duobinaryx_(n) + x_(n-1) Dicode x_(n) − x_(n-1) Modified Duobinary x_(n) −x_(n-2) Class 2 x_(n) + 2x_(n-1) + x_(n-2)

Unless otherwise described, as used herein the duobinary processingperformed is assumed to be a summation of the present and immediatelyprevious transmit unit interval signal, each scaled by a factor of 0.5.Optionally, this may be combined with a transmit lowpass filter tofurther control the transmit spectrum. In other embodiments,ISI-controlling encoding is combined in any order with Hadamardencoding, where the ISI-controlling encoding is any of duobinary,modified duobinary, dicode, class2, or a Hamming filter as subsequentlydescribed. In such embodiments, the ISI-controlling encoding may also bedescribed as being performed by a partial response encoder, embodyingany of the partial response encodings or filterings above.

If the characteristics of the communications channel are extremely wellunderstood, it may be possible to configure the ISI-controllingoperation of the transmitter such that no explicit complementaryoperation is required at the receiver, the effective action of thechannel characteristics themselves serving to perform the inverseoperation. Other embodiments may explicitly detect, as one example, theternary signals produced by duobinary encoding of binary data, followedby an explicit duobinary to binary decoding operation. Alternatively,commonly used receiver ISI elimination techniques such as DFE will alsoefficiently address the effects of such transmitter ISI compensation. Asexample receivers in this document incorporate DFE, no further receiverduobinary (or other partial response code) processing will be shown.

Channelization

Physical transport channel limitations have been seen and addressedbefore, albeit at far lower data rates, during the efforts to providehigh speed digital services over the legacy copper wire infrastructureof the telephony network. For DSL at its desired 3 Megabit data rate, apropagating signal wavelength was several hundred meters, whichcorrelated strongly with the typical spacing of wire stubs, splices, andinsulation abrasions seen in the field. Thus, an uncompensated frequencyresponse for a typical copper telephony signal path would exhibitnumerous notches and slopes caused by reflective interference amongthose anomalies, dissipative attenuation from degraded wires andinsulation, and intrusive noise from sources such as AM radiotransmitters.

Ultimately, multichannel frequency domain channelization was used toconstrain the effect of those legacy transport issues. One commonlydeployed Asymmetric Digital Subscriber Line (ADSL) solution, forexample, partitioned the approximate 1 MHz of available transport mediumbandwidth into 4.3125 kHz channels. Each channel was then independentlytested for attenuation and signal-to-noise ratio, with different datathroughput rates assigned to each channel depending on those testresults. Thus, a channel frequency coinciding with a frequency responsenotch or significant external noise source would not be used, whileother channels not presenting those issues could be used at fullcapacity. Unfortunately, the generation and detection of such a highchannel count protocol relies on the availability of inexpensive digitalsignal processing solutions, and such technology has scaled inperformance over time by perhaps a factor of ten, versus the approximatefactor of 100,000 data rate increase in the present application.

Thus, although the present channel attenuation issues suggest abroadband approach may be useful, the conventional high-channel-countembodiment methods known to the art are incompatible with theanticipated data rate. A new approach specifically designed for highspeed processing will be described.

Broadband Duobinary ENRZ

[Hormati III] gives examples of several embodiments combining ENRZsignaling with an additional serial transmission encoding such asDuobinary, utilizing multiple frequency-domain channels. Those examplesand teachings are incorporated by reference herein, in their entiretyfor all purposes.

FIG. 1 is a block diagram of a further receiver embodiment that will beused to illustrate the following examples. In the receiver of FIG. 1,four communications wires w0-w3 carry a mixture of baseband andbroadband signals; a first frequency channel is at baseband, i.e.comparable to a typical wire communication channel known in the art. Thesecond frequency channel is herein called the “carrier”,“carrier-modulated”, or “broadband” channel, and is composed ofENRZ+duobinary signaling modulating a sinusoidal carrier, chosen tominimize the frequency overlap between spectral components of thebaseband and of the carrier channel.

As in [Hormati III], a carrier frequency of 37.5 GHz is assumed. Bothbaseband and carrier channels run at a signaling rate of 37.5Gsymbols/second, with a first set of three data bits being transportedover the four wires of the baseband channel, and a second set of threedata bits being transported over the same four wires using the carrierchannel.

Other embodiments are known in which the baseband signaling rate differsfrom the carrier signaling rate, and/or differ from the carrierfrequency. Generally speaking, however, there are implementationadvantages in keeping these relationships fixed, often being expressedas ratios of small integer values, as in the 1:1:1 example providedabove. As one example of such advantage, a receiver embodiment may thenmaintain a single local oscillator clock derived from one such receivedsignal, and then derive the other necessary receive clocks from itthrough known phase lock or delay lock methods.

Filters 110 and 115 separate the received signals into a broadbandcomponent including a set of carrier-modulated symbols of acarrier-modulated codeword and a baseband component including a set ofbaseband symbols of a baseband codeword. For descriptive simplicity,FIG. 1 includes a High-pass filter 110 to generate the carrier-modulatedsymbols and a Low-pass filter 115 to generate the baseband symbols,although in practice band-pass or band-rejection filters may also beapplicable. Following the baseband signal flow from the output of filter115, so-called Multi-Input Comparators (MICs) 130 as per [Holden I] and[Ulrich I] perform weighted linear combinations of the various wiresignals represented as baseband symbols, producing outputs which detectthe individual subchannel data outputs of the ENRZ code. MICs 130 mayadditionally perform a slicing or time and amplitude sampling ormeasurement of each subchannel data output under control of a samplingclock provided by Clock/Data Recovery (CDR) subsystem 165.

As shown in FIG. 1, different system considerations may includedifferent synchronization sources for CDR 165. In a first embodiment(1), a data-sampling clock is derived from data transitions of thedetected subchannel data itself, using methods as described in [HormatiIV]. In a second embodiment (2), a subchannel of the ENRZ basebandsignal provides a synchronization source, either by dedication to aperiodic clock signal, or by augmentation of the transition density of adata stream to guarantee sufficient edge information to maintain clocksynchronization. A third embodiment (3) uses a transmitted clock orsynchronization signal distinct from the ENRZ data to synchronize theCDR subsystem, which may be transmitted from the transmitter to thereceiver via a dedicated differential pair of wires.

As with the baseband data path, the broadband data detection pathincorporates MIC-demodulation circuits 120, samplers 127, and CDR 150functions. However, as broadband encoded data is modulated onto acarrier, simple data detection cannot be performed without addressingthe carrier signal as well.

As is well understood in the art, a signal modulated on a carrier may bemixed with a demodulations signal provided by e.g., a local oscillator,to return the carrier-modulated signals to baseband (a heterodynereceiver,) or data detection may be timed relative to not only datasampling rate, but also relative to the carrier rate (a synchronousdetector.) In FIG. 1, such an operation is performed by MIC-Demodulationcircuits 120, as will be subsequently described. The demodulation signalmay be generated by frequency multiplier 160, which in this exampleoffered without limitation derives a carrier frequency reference from asampling clock signal generated by baseband CDR 165. The detectedsubchannel information is processed using a Low Pass Filter 125, toremove residual artifacts of the heterodyne or synchronous detectionoperation of MIC-demodulation circuits 120. Carrier CDR subsystem 150produces a data-sampling clock appropriate for the demodulated broadbandchannel data sampling 127. Depending on the particular demodulationmethod used by 120, Demodulation clock manager 140 may provide ademodulation signal as generated by frequency multiplier 160, a datasampling clock as provided by 150, or a combination of clocks fordemodulation.

Alternative embodiments may synchronize a receiver clock to the carrierand derive other sampling clocks from that derived reference source, maysynchronize a receiver clock to a detected data stream and derive othersampling clocks from that derived reference source, or utilize acombination of said methods. Synchronization may utilize a local voltagecontrolled oscillator (VCO) or voltage controlled delay in aphase-locked or delay-locked loop generating a local clock signal.Alternatively, synchronization may rely upon utilizing a phasecomparator result that configures a phase interpolator or adjustabledelay to modify the phase of a local clock signal.

Synchronously-Switched Multi-Input Demodulating Comparator

FIG. 2 is a schematic showing one embodiment of a linear mode ENRZdetector. Input signals w0, w1, w2, w3 are the received wire signalscarrying the low-pass filtered (not shown) baseband symbols of thebaseband codeword, and are connected to the transistors in accordancewith a particular subchannel of the ENRZ code. That is, positive valuesof the subchannel vector indicate that the wire is connected to atransistor contributing to an output in a positive manner, whilenegative values indicate that the respective wire is connected to atransistor contributing to a negative portion of the output. Thus, inFIG. 2, the wires are connected to the transistors in accordance withthe subchannel vector [1, −1, 1, −1], and the differential outputs QHand QL correspond to the result of the linear combination generated bythe computation

Q=(w0+w2)−(w1+w3)   (Eqn. 2)

As described in [Holden I], three instances of Eqn. 2 with differentpermutations of the input signals efficiently detect the threesubchannel data outputs of the ENRZ code. Thus, the baseband detector130 of FIG. 1 may be composed of three instances of the circuit of FIG.2.

FIG. 3 is an embodiment of a synchronously-switched demodulating MICcircuit (also referred to herein as a “MIC-demodulation circuit”)implementing the same linear combination of the MIC in FIG. 2, in whichthe demodulation signal CK_d operating at the carrier frequency is usedto gate operation of the MIC-demodulation circuit, allowing synchronousdetection of carrier-modulated symbols of the carrier-modulatedcodeword. In a practical embodiment, CK_d will be phase locked to thecarrier frequency provided by frequency multiplier 160 in FIG. 1. Asshown in FIG. 3, the MIC-demodulation circuits obtains a set ofcarrier-modulated symbols of a carrier-modulated codeword. Eachcarrier-modulated symbol may be received via a respective wire of aplurality of wires of a multi-wire bus, and may have been high-pass (orband-pass) filtered prior to being applied to a corresponding transistorof the set of transistors shown. Each transistor is connected to arespective output node of a pair of output nodes according to elementsof a sub-channel vector. For example, in the example shown in FIG. 3,the set of transistors connected to wires [w0 w1 w2 w3] are connected tothe pair of output nodes QL and QH according to the sub-channel vector[1 −1 1 −1], which corresponds to the second row of the H4 Hadamardmatrix given in Eqn. 1. The sets of transistors making up the MICs forthe other two sub-channels may be connected to pairs of output nodesaccording to the other sub-channel vectors in the H4 Hadamard matrixthat are mutually orthogonal to the [1 −1 1 −1] sub-channel vector. Insome further embodiments, each transistor in the set of transistors mayapply a respective magnitude weight to the carrier-modulated symbolaccording to the sub-channel vector. Eqn. 3 below is one particularmatrix that includes sub-channel vectors having various magnitudes.Magnitude weights may be applied e.g., according to relative transistordimensions, current source magnitudes, and/or multiple equal-sizedtransistors connected in parallel. The conductivity of the set oftransistors is synchronously switched, and is controlled according tothe demodulation signal CK_d, which is operating at a frequencyrecovered from the carrier-modulated symbols to responsively generate ademodulated sub-channel data output as a linear combination performedaccording to Eqn. 2 of the set of carrier-modulated symbols, which formsa differential voltage on the pair of output nodes.

$\begin{matrix}\begin{bmatrix}1 & 1 & 1 & 1 & 1 & 1 \\1 & {- 1} & 0 & 0 & 0 & 0 \\1 & 1 & {- 2} & 0 & 0 & 0 \\0 & 0 & 0 & 1 & {- 1} & 0 \\0 & 0 & 0 & 1 & 1 & {- 2} \\1 & 1 & 1 & {- 1} & {- 1} & {- 1}\end{bmatrix} & ( {{Eqn}.\mspace{14mu} 3} )\end{matrix}$

FIG. 4 is a further embodiment of a MIC-demodulation ENRZ detector. TheMIC-demodulation circuit operates in a dynamic mode, charging the nodecapacitance of the pair of output nodes QH and QL when a sampling clockCK operating at the symbol rate is low, and selectably providing adischarge path for those nodes through the set of input transistors toground when CK is high. As the nodes discharge, a differential outputsignal corresponding to the result of Eqn. 2 is produced at output nodesQH and QL. In such an embodiment, the sampling clock simultaneously actsas the demodulation signal as the carrier-modulated symbols aredemodulated and the linear combination is sliced according to thesampling clock. Such an embodiment may occur when the sampling clockrate and the carrier frequency are equal. In some alternativeembodiments, as shown, synchronous switching may be achieved using ademodulation signal CK_d to periodically interrupt or gate the dynamicoperation of the detector at a carrier frequency rate, allowing directdetection of signals modulating a carrier. In some such embodiments, thedemodulation signal CK_d may be an integer multiple of the basebandsymbol baud rate of sampling clock CK, e.g., 2×, 3×, etc. Alternatively,the demodulation signal may have a frequency that is a fraction of thesampling clock rate.

FIG. 5 is a full-wave variation of the half-wave synchronous detector ofFIG. 3. As with the half-wave detector, synchronously-switched ENRZdetection is gated by action of demodulation signal CK_d+ and itsinverse or complement CK_d− which are phase locked to the carrierfrequency. In an alternative embodiment, each differential pair oftransistors may be composed of an NMOS device and a PMOS device thatboth receive the same demodulation signal CK_d+. In this full-wavecircuit variation, the contribution of each input to the pair of outputnodes is steered on alternating half cycles of the clock to theinverting and non-inverting result outputs, effectively doubling theoutput signal and substantially reducing the need for output filtering.The steering is performed by a plurality of differential pairs oftransistors connected to the pair of output nodes for selectablyconnecting each transistor of the set of transistors to a respectiveoutput node according to the demodulation signal and its inverse. Asshown, the set of transistors receiving the carrier-modulated symbolsare alternately connected between the pair of output nodes viarespective differential pairs of transistors. For each sub-channel MIC,the differential pairs of transistors may be connected according to therespective sub-channel vector of the plurality of mutually orthogonalsub-channel vectors. In the case of FIG. 5, the differential pairs oftransistors are connected to the pair of output nodes according to thesub-channel vector [1 −1 1 −1].

FIG. 6 illustrates a discrete full-wave variation of the discretehalf-wave dynamic synchronous detector of FIG. 4. Sampling clock CKcontrols the dynamic charge/discharge operation of the circuit, whiledemodulation signal CK_d+ and its complement CK_d− gate detection to besynchronous with the carrier frequency, as described above.

In the examples of FIGS. 4 and 6, the timing of the two clock signalsmay be coordinated so as to be compatible with the dynamic circuitoperation. Specifically, in FIG. 4 a pre-charge operation occurring whenclock CK is low may precede each discharge or integration cycleoccurring when clocks CK_d and CK are both high. In FIG. 6, a full cycleof clock CK (i.e. a pre-charge cycle and a discharge cycle) may occurfor each half-cycle of CK_d. Alternatively, the integration time for thecircuit may be long enough to encompass two or more half-cycles of CK_d.In a further alternative embodiment, the demodulation signal CK_d+/− maybe an integer multiple of CK. In such embodiments, multiple cycles ofCK_d essentially gate the signal into or out of the integration period.In such embodiments, the pair of output nodes are pre-charged accordingto the sampling clock, and the pair of output nodes are only dischargedwhen the carrier-modulated symbols at the inputs of the transistors havethe correct polarity. In the full-wave embodiment, the pair of outputnodes are always discharged, but the connections of the set oftransistors receiving the carrier-modulated symbols to the pair ofoutput nodes is alternated according to the demodulation signal as thecarrier-modulated symbols alternate according to the modulation.

Operation of the half-wave and full-wave synchronous detectors isillustrated in the waveforms of FIG. 7. As described in [Hormati III], atransmitter multiplies Transmit Data and a Carrier-rate clock to producea carrier-modulated signal. A receiver locally generates a carrier-ratedemodulation clock, which is combined with the received signal in asynchronous detector to produce a detectable receive data signal. Thecarrier-modulated signal is encoded into carrier-modulated symbols of acarrier-modulated codeword and transmitted over the wires of themulti-wire bus.

The illustrated Return-to-Zero (RTZ) waveform in FIG. 7 is one exampleof an output of a half-wave detector circuit, while the Full-wavewaveform is one example of an output of a full-wave detector circuit. Insome embodiments, both such outputs are low-pass filtered to aid inreconstituting the detected waveform and eliminating spurious signalartifacts. Each waveform may then be sampled in the center of the datainterval, as illustrated by the vertical hash marks.

FIG. 8A is a block diagram illustrating a conventional multiple PLLapproach to generating the necessary receive clocks for an embodimentsuch as shown in FIG. 1. Using information obtained directly orindirectly from the received data (which may include transmitted clocksvia dedicated wires, a dedicated sub-channel, and/or enhanced edgetransition density, eye width or edge measurement, etc.)appropriately-timed sampling clocks are generated to permit optimumsampling of detected baseband data. This is commonly known as Clock-DataAlignment or Clock-Data Recovery (CDA or CDR). In common embodiments, aPLL configuration is used in which a phase detector controls a VoltageControlled Oscillator (VCO) so as to generate a local clock signalhaving the desired characteristics.

Separately, an independent PLL configuration generates a local clocksignal aligned to the carrier frequency of the receivedcarrier-demodulated data, and a sampling clock suitable for optimumsampling of carrier-demodulated data.

In an alternative embodiment, one or more of the necessary local clocksignals may be derived from another clock signal. FIG. 8B illustrates alocal clock signal created using a PLL locked to a reference derivedfrom the Baseband Data. This may be facilitated by, as representativeexamples, the Baseband data incorporating a dedicated clock signal, orbeing augmented by a guaranteed density of edge transitions. Thiswell-controlled and stable local clock may then be used to generateother local clock signals, either by well-known frequency division ormultiplication (e.g., using frequency multiplier 160 of FIG. 1), orsimply by adjusting the relative phase of the derived clock using aphase interpolator or adjustable delay element (e.g., using offsetcorrection element 140). For example, the local clock signal derivedfrom baseband data may have the same frequency as the carrier frequency,and may thus be phase-adjusted using a phase detector and phaseinterpolator to generate the demodulation signal. Further, if the datarate of the demodulated sub-channel data is equal to the data rate ofthe baseband data, the clock derived from the baseband data may drivegeneration of the sampling clock for the demodulated sub-channel data,and may be phase-shifted accordingly. In some embodiments, the carrierfrequency is different than the data rate of the baseband data, and thusthe sampling clock generated from the baseband data may bemultiplied/divided (not shown) to generate a demodulation signal havingthe carrier frequency (as is the case shown in FIG. 1). The demodulationsignal may then be phase-detected and phase-shifted to be aligned to thecarrier-modulated symbols. FIG. 8B shows two such derived clocks, eachbeing phase-adjusted by a separate phase detector and phaseinterpolator. In some embodiments, the phase-adjustments are made tocompensate for differences in the signal paths for the baseband/carriersignals, as they undergo different filtering, etc.

In some embodiments, hybrid clock generation embodiments are alsopossible, including embodiments utilizing secondary or slave PLLs thatproduce a second local clock that is derived from a first local clockgenerated as described above. In such a system configuration, thesecondary PLL may have different lock characteristics than the primaryPLL, allowing independent optimization of characteristics such as locktime, free-running drift, jitter, etc.

FIG. 9 is a flowchart of a method 900, in accordance with someembodiments. As shown, method 900 includes obtaining 902 a set ofcarrier-modulated symbols of a carrier-modulated codeword, eachcarrier-modulated symbol received via a respective wire of a pluralityof wires of a multi-wire bus. Each carrier-modulated symbol of the setof carrier-modulated symbols is applied 904 to a correspondingtransistor of a set of transistors, the set of transistors connected toa pair of output nodes according to a sub-channel vector of a pluralityof sub-channel vectors. A demodulation signal CK_d is recovered 906 fromthe set of carrier-modulated symbols. A demodulated sub-channel dataoutput is generated 908 as a differential voltage on the pair of outputnodes based on a linear combination of the set of carrier-modulatedsymbols by controlling a conductivity of the set of transistorsaccording to the demodulation signal CK_d.

In some embodiments, controlling the conductivity of the set oftransistors includes selectably enabling a current source according tothe demodulation signal CK_d, as shown in FIGS. 3 and 4. In suchembodiments, the demodulated sub-channel data output is a return-to-zero(RTZ) signal, as illustrated in FIG. 7.

In some embodiments, controlling the conductivity further includesalternately connecting each transistor of the set of transistors betweenthe pair of output nodes according to the demodulation signal. In suchembodiments, each transistor circuit is alternately connected betweenthe pair of output nodes using a respective differential pair oftransistors connected to the pair of output nodes according to thesub-channel vector. In some embodiments, the respective differentialpair of transistors receives the demodulation signal and a complement ofthe demodulation signal and is composed of same-type transistors (e.g.,NMOS or PMOS only), while alternative embodiments may includedifferential pairs of transistors that include an NMOS and a PMOStransistor both receiving the demodulation signal CK_d. Such aconfiguration is illustrated in FIGS. 5 and 6, as the differential pairsreceiving the demodulation signal CK_d+ and a complement of thedemodulation signal CK_d− alternate the connection of the correspondingtransistor connected to the carrier-modulated symbols between eachoutput node in accordance with the polarity of the carrier-modulatedsymbols.

In some embodiments, the conductivity of each transistor of the set oftransistors is further controlled by a symbol value of the appliedcarrier-modulated symbol. In such embodiments, the amount of currentdrawn through each transistor is proportional to the symbol valueapplied at each transistor. Signal amplitudes in one particularembodiment are:

500 mV center,

500+180=680 mV (+1)

500−60=440 mV (−1/3)

500−60=440 mV (−1/3)

500−60=440 mV (−1/3)

where a symbol of magnitude ‘1’ corresponds to a 180 mV deviation fromthe 500 mV center voltage and a symbol of magnitude ‘1/3’ corresponds toa 60 mV deviation from the 500 mV center voltage.

In some embodiments, the method further includes pre-charging the pairof output nodes in response to a sampling clock, and wherein theconductivity of the set of transistors is further controlled accordingto the sampling clock. Such embodiments are referred to above as“discrete” or “dynamic” MIC-demodulation circuits.

In some embodiments, the differential voltage on the pair of outputnodes is formed by drawing currents through impedance elements connectedto the pair of output nodes. In some embodiments, the impedance elementsmay be resistors connected between a power supply and the pair of outputnodes to control a voltage drop across the resistors. The differentialamount of current drawn through the resistors will form a differentialvoltage output on the pair of output nodes.

In some embodiments, the method further includes low-pass filtering thedemodulated sub-channel data output.

In some embodiments, obtaining the set of carrier-modulated symbolsincludes high-pass filtering a superposition codeword comprising the setof carrier-modulated symbols of the carrier-modulated codeword and a setof baseband symbols of a baseband codeword.

In some embodiments, the sub-channel vector is part of a plurality ofmutually orthogonal sub-channel vectors that compose rows of anorthogonal matrix. In some such embodiments, the orthogonal matrix is aHadamard matrix.

In some embodiments, the demodulation signal has an equal rate as asampling clock associated with a baud rate of the data streams. Inalternative embodiments, the demodulation signal has a differing ratethan the sampling clock. In some such embodiments, the demodulationsignal may be an integer multiple of the sampling clock, and mayinitiate multiple discharge periods in a single unit interval.Alternatively, the demodulation signal may be a fraction of the samplingclock. In such embodiments, the sampling clock may initiate multiplepre-charge/discharge cycles in a half cycle of the demodulation signal.In some embodiments, the demodulation signal is generated by multiplyingthe sampling clock using a frequency multiplier. Alternatively, thedemodulation signal may be generated by dividing the sampling clockusing a frequency divider.

1. A method comprising: obtaining a baseband clock signal; receivingwire signals over a multi-wire bus, the wire signals comprising asuperposition of a set of baseband symbols and a set ofcarrier-modulated symbols; sampling, using the baseband clock signal, aset of baseband sub-channel outputs generated from combining the set ofbaseband symbols; deriving a phase-adjusted demodulation signal from thebaseband clock signal, the phase-adjusted demodulation signal having afrequency equal to a carrier frequency associated with the set ofcarrier-modulated symbols; generating a set of demodulated sub-channeloutputs by synchronously forming linear combinations of the set ofcarrier-modulated symbols responsive to the phase-adjusted demodulationsignal; generating a carrier band data sampling clock signal byphase-adjusting the baseband clock signal according to the set ofdemodulated sub-channel outputs; and sampling the set of demodulatedsub-channel outputs using the carrier band data sampling clock signal.2. The method of claim 1, wherein the baseband clock signal is obtainedfrom a dedicated pair of clock wires of the multi-wire bus.
 3. Themethod of claim 1, wherein the set of baseband sub-channel outputscomprises a dedicated clock sub-channel output that is orthogonal to aplurality of data sub-channel outputs, and wherein the baseband clocksignal is derived from the dedicated clock sub-channel output.
 4. Themethod of claim 1, wherein obtaining the baseband clock signal comprisesdetecting signal transitions in the set of baseband sub-channel outputs.5. The method of claim 4, wherein the baseband sub-channel outputs areencoded to maintain a guaranteed minimum transition density.
 6. Themethod of claim 1, wherein deriving the phase-adjusted demodulationsignal from the baseband clock signal comprises frequency-multiplyingthe baseband clock signal to achieve the frequency equal to the carrierfrequency associated with the set of carrier-modulated symbols.
 7. Themethod of claim 1, wherein deriving the phase-adjusted demodulationsignal from the baseband clock signal comprises frequency-dividing thebaseband clock signal to achieve the frequency equal to the carrierfrequency associated with the set of carrier-modulated symbols.
 8. Themethod of claim 1, wherein deriving the phase-adjusted demodulationsignal comprises phase-adjusting the baseband clock signal to be alignedto the set of carrier-modulated symbols.
 9. The method of claim 1,wherein generating each demodulated sub-channel output of the set ofdemodulated sub-channel outputs comprises generating a differentialvoltage on a pair of output nodes based on a differential current formedfrom the linear combination of the set of carrier-modulated symbols, thedifferential current selectively enabled according to the phase-adjusteddemodulation signal.
 10. The method of claim 9, wherein each linearcombination of the set of carrier-modulated symbols is mutuallyorthogonal.
 11. An apparatus comprising: a set of multi-inputcomparators (MICs) configured to receive wire signals in parallel over amulti-wire bus, the wire signals comprising a superposition of a set ofbaseband symbols and a set of carrier-modulated symbols, the set ofmulti-input comparators comprising: a set of baseband MICs configured togenerate a set of baseband sub-channel outputs by combining the set ofbaseband symbols; and a set of demodulation MICs configured to generatea set of demodulated sub-channel outputs by synchronously forming linearcombinations of the set of carrier-modulated symbols responsive to aphase-adjusted demodulation signal; a clock recovery circuit comprising:a voltage-controlled oscillator (VCO) configured to generate a basebandclock signal; a first phase-locked loop (PLL) configured to generate thephase-adjusted demodulation signal from the baseband clock signal, thephase-adjusted demodulation signal having a frequency equal to a carrierfrequency associated with a set of carrier-modulated symbols; and asecond PLL configured to generate a carrier band data sampling clocksignal by phase-adjusting the baseband clock signal according to the setof demodulated sub-channel outputs; a plurality of samplers configuredto sample the set of baseband sub-channel outputs and the set ofdemodulated sub-channel outputs according to the baseband clock signaland the carrier band data sampling clock signal, respectively.
 12. Theapparatus of claim 11, wherein the VCO is configured to generate thebaseband clock signal based on a forwarded clock signal received over adedicated pair of clock wires of the multi-wire bus.
 13. The apparatusof claim 11, wherein the set of baseband sub-channel outputs comprises adedicated clock sub-channel output that is orthogonal to a plurality ofdata sub-channel outputs, and wherein the VCO is configured to generatethe baseband clock signal based on the dedicated clock sub-channeloutput.
 14. The apparatus of claim 11, wherein the VCO is configured togenerate the baseband clock signal based on signal transitions in theset of baseband sub-channel outputs.
 15. The apparatus of claim 14,wherein the baseband sub-channel outputs are encoded to maintain aguaranteed minimum transition density.
 16. The apparatus of claim 11,wherein the clock recovery circuit further comprises afrequency-multiplier configured to frequency-multiply the baseband clocksignal to have the frequency equal to the carrier frequency associatedwith the set of carrier-modulated symbols.
 17. The apparatus of claim11, wherein the clock recovery circuit further comprises afrequency-divider configured to frequency-divide the baseband clocksignal to have the frequency equal to the carrier frequency associatedwith the set of carrier-modulated symbols.
 18. The apparatus of claim11, wherein the first PLL is configured to generate the phase-adjusteddemodulation signal by phase-adjusting the baseband clock signal to bealigned to the set of carrier-modulated symbols.
 19. The apparatus ofclaim 11, wherein each demodulation MIC is configured to generate arespective demodulated sub-channel output of the set of demodulatedsub-channel outputs by generating a differential voltage on a pair ofoutput nodes based on a differential current formed from the linearcombination of the set of carrier-modulated symbols, the differentialcurrent selectively enabled according to the phase-adjusted demodulationsignal.
 20. The apparatus of claim 19, wherein each linear combinationof the set of carrier-modulated symbols is mutually orthogonal.